Our 16-year-old lab mix is exhibiting repetitive and compulsive behavior, such as scratching ALL NIGHT on our doors, digging in the yard, pacing, excessive panting, etc. Some or all of this behavior can be attributed to "doggy alzheimer's." She is losing her hearing and eyesight, has experienced severe weight loss even though she eats, and has labored movement. She also has large growths all over her body, which our vet said would be pointless to operate on because of her age. Our 3-year-old miniature pinscher seems to keep her going with his endless supply of energy. She actually runs from time to time like she did when she was younger. Most of the time she seems discontented and is ONLY happy when she is lying inside on "her spot." My question then is when is it appropriate to put down a very old, seemingly unhappy dog?

Most people would say put a dog down when it is suffering/hurting. It seems like your dog is getting old, senile, maybe grumpy, but from what I read she is not in much pain. My advice is to keep her around, she doesn't seem to have a lot of years left anyway, but as long as the dog isn't in pain, I wouldn't think you should put her down.

Is she on medication for the "doggy alzheimers"? If not, it could make a big difference in her mood.
Otherwise, I think you have to trust your instincts about her quality of life; you know her better than anyone else.

I also want to add that, we as animal lovers. Wants to hold on to our pets as long as we can. My sammy was 18 years old. Had to be put in pain because one morning she just started yelping in pain and that was it. The thing is you wait too long is not a good think either. I think that is a great suggestion above, maybe making her comfortable by improving her moods will improve on her life. At the same time it is your decision in the end of it. It is very tough choice to make. Do not let it get to the point when she is in lots of pain.

I have a 15 year old Min.Pin. And a 14 year old Doxie, my Min.Pin. Had a stroke on May 4th this year,she nearly died.I was ready to put her down because she could not walk at all.She was not in pain but to me if a dog can't be a dog then its time to go.She walked on Mothers Day all by herself,so the therapy paid off... she is a fighter and God knows she's been through it.. you will know when it's time to let her go.. when she can't be herself she will let you know... good luck to you...

My 2 cents... I went through this last year, it's so hard. You love your dog and want them with you, but you don't want to be selfish in your decision either. It's a very personal decision. I always looked at the quality of life. Dogs don't always yelp or cry when in pain, my dog was deaf and blind. I kept him around the last year because there were certain things he still really enjoyed, car rides, ear rubs, his treats, and a good rub down in bed when we woke each morning. Slowly I noticed his enjoyment diminishing and he couldn't chew his favorite treats anymore because of bad teeth and certain mouth pain. He became stiff each morning and seemed so tender when I rubbed and petted him. For me this was the time, his quality of life was gone. Again, it's such a personal and difficult decision, my heart goes out to you both.